The overall goal of these studies is to test the feasibility of, and lay the foundations for, specific immunosuppressive treatments of factor VIII (fVIII) antibody inhibitors. We propose to develop and test in hemophilia A mice (they have a disrupted fVIII gene) procedures for "tolerization" of the CD4+ T helper cells involved in the synthesis of fVIII antibody inhibitors. Similar procedures were very effective in preventing and down regulating another Ab-mediated mouse syndrome, experimental myasthenia gravis. Also, we will investigate the epitope repertoire of human anti-fVIII CD4+ cells. If tolerization procedures will be effective in mice, knowledge of the CD4+ repertoire in humans will be needed for development of similar treatments of fVIII inhibitors for hemophilia A patients. The specific aims will be: 1) To determine the epitope repertoire of anti-fVIII CD4+ cells in hemophilia A mice immunized with fVIII. We will challenge CD4+ spleen cells of fVIII immunized mice with overlapping synthetic peptides spanning the fVIII sequence. 2) To determine whether the anti-fVIII CD4+ cells in hemophilia A mice are Thl or Th2. These experiments will determine whether Thl or Th2 cells, or both, are necessary for synthesis of anti-fVIII Ab in general, and inhibitors in particular. This information will help the design of optimal tolerization procedure. 3) To use synthetic fVIII peptides comprising CD4+ epitope sequences, for tolerization procedures in hemophilia A mice. We will first test if nasal tolerization procedures will affect development of CD4+ and Ab response to fVIII. We will administer in the nasal cavities of the mice solutions of the synthetic fVIII CD4+ epitope peptides, prior to and during immunization with fVIII. If nasal administration will not be effective, we will attempt subcutaneous administration of the same peptides. 4) To investigate the epitope repertoire of human anti-fVIII CD4+ cells. We will challenge with the overlapping synthetic fVIII sequences used for the mice studies, blood CD4+ T cells from hemophilia A patients with and without antibody inhibitors, acquired autoimmune hemophilia patients and healthy subjects, which have ephemeral yet clearly detectable CD4+ responses to fVIII.